In recent years, due to the requirement of reduction in cost for drilling of oil wells, construction methods have been developed in which pipe expansion is performed in a well using an expanding process (see, for example, PCT Japanese Translation Patent Publication No. 7,507,610 and WO 98/00626). Hereinafter, this construction method is called a “solid expandable tubular system.” According to this solid expandable tubular system, a casing is expanded radi-ally in a well. Compared to a conventional construction method, when the same well radius is to be ensured, each of the diameters of individual sections forming a casing having a multistage structure can be decreased. In addition, since the size of a casing for an exterior layer of an up-per portion of the well can also be decreased, the cost for drilling a well can be reduced.
In the solid expandable tubular system described above, since being exposed to oil or gas environment immediately after a expanding process is carried out, steel pipes thus formed are not processed by heat treatment after the process described above, and hence the steel pipes are required to have corrosion resistance as cold expanded. In order to satisfy the requirement described above, Japanese Unexamined Patent Application Publication No. 2002-266055 discloses expandable oil country tubular goods having superior corrosion resistance after a expanding process. JP '055 discloses the expandable oil country tubular goods comprising 0.10% to 0.45% of C, 0.1% to 1.5% of Si, 0.10% to 3.0% of Mn, 0.03% or less of P, 0.01% or less of S, 0.05% or less of sol. Al, and 0.010% or less of N are contained on a mass percent basis, the balance being composed of Fe and impurities. JP '055 discloses a steel pipe, in which the strength (yield strength YS (MPa)) before a expanding process and the crystal grain diameter (d(μm)) satisfy an equation represented by ln(d)≦−0.0067YS+8.09. In addition, it has also been disclosed that, in the same steel pipe described above, (A) at least one of 0.2% to 1.5% of Cr, 0.1% to 0.8% of Mo, and 0.005% to 0.2% of V on a mass percent basis, (B) at least one of 0.005% to 0.05% of Ti and 0.005% to 0.03% of Nb on a mass percent basis, and (C) at least one of 0.001% to 0.005% of Ca are contained instead of a part of the Fe.
In addition, Japanese Unexamined Patent Application Publication No. 2002-349177 discloses that, in order to prevent the decrease in collapse strength caused by the increase in rate of wall-thickness deviation by pipe expansion, the rate of wall-thickness deviation EO (%) before pipe expansion is controlled to be 30/(1+0.018α) or less (where a (expand ratio)=(inside diameter after pipe expansion/inside diameter before pipe expansion−1)×100). In addition, in order to prevent a steel pipe from being bent which is caused by the conversion of the difference in expansion amount in the circumferential direction to the difference in contraction amount in the longitudinal direction, JP '177 discloses that the rate of eccentric wall-thickness deviation (primary wall-thickness deviation) (%) (={(maximum wall thickness of a component of eccentric wall-thickness deviation−minimum wall thickness thereof)/average wall thickness}×100) is controlled to be 10% or less.
According to JP '055 and JP '177, a preferable manufacturing method has been disclosed in which quenching and tempering are performed for electric resistance welded steel pipes or seamless steel pipes obtained after pipe forming or in which quenching is repeatedly performed therefor at least two times, followed by tempering, and an example has been disclosed in which a expanding process is performed within an expand ratio of 30% or less.
Due to further cost reduction needs, inexpensive steel pipes have been desired which can withstand an expanding process performed at a high expansion ratio, such as more than 30%. When a steel pipe can be expanded in a well at an expansion ratio larger than a conventional value of 30%, the size of casing can be further decreased and, hence, the drilling cost can be further decreased. In order to satisfy the need described above, it would be advantageous to provide seamless expandable oil country tubular goods, which have excellent pipe-expansion properties capable of withstanding an expanding process at an expansion ratio of more than 30% although having a high strength, such as a tensile strength (TS) of 600 MPa or more, and a manufacturing method thereof. In addition, unlike the case disclosed in JP '055 and JP '177, without receiving quenching and tempering (Q/T) treatment, the seamless expandable oil country tubular goods described above should be in an as-rolled state or processed by nonthermal-refining type heat treatment (normalizing (annealing) treatment or dual-phase heat treatment) which is a less expensive heat treatment.